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    <h5>PolygonUpdates class</h5>
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      <div><h1><span class="kind-constructor">PolygonUpdates.from</span> constructor </h1></div>

    <section class="multi-line-signature">
      
      <span class="name ">PolygonUpdates.from</span>(<wbr><ol class="parameter-list"><li><span class="parameter" id="from-param-previous"><span class="type-annotation"><a href="https://api.flutter.dev/flutter/dart-core/Set-class.html">Set</a><span class="signature">&lt;<wbr><span class="type-parameter"><a href="../../amap_flutter_map/Polygon-class.html">Polygon</a></span>&gt;</span></span> <span class="parameter-name">previous</span>, </span></li>
<li><span class="parameter" id="from-param-current"><span class="type-annotation"><a href="https://api.flutter.dev/flutter/dart-core/Set-class.html">Set</a><span class="signature">&lt;<wbr><span class="type-parameter"><a href="../../amap_flutter_map/Polygon-class.html">Polygon</a></span>&gt;</span></span> <span class="parameter-name">current</span></span></li>
</ol>)
    </section>

    <section class="desc markdown">
      <p>通过Polygon的前后更新集合构造一个PolygonUpdates</p>
    </section>
    
    <section class="summary source-code" id="source">
      <h2><span>Implementation</span></h2>
      <pre class="language-dart"><code class="language-dart">PolygonUpdates.from(Set&lt;Polygon&gt; previous, Set&lt;Polygon&gt; current) {
  if (previous == null) {
    previous = Set&lt;Polygon&gt;.identity();
  }

  if (current == null) {
    current = Set&lt;Polygon&gt;.identity();
  }

  final Map&lt;String, Polygon&gt; previousPolygons = keyByPolygonId(previous);
  final Map&lt;String, Polygon&gt; currentPolygons = keyByPolygonId(current);

  final Set&lt;String&gt; prevPolygonIds = previousPolygons.keys.toSet();
  final Set&lt;String&gt; currentPolygonIds = currentPolygons.keys.toSet();

  Polygon idToCurrentPolygon(String id) {
    return currentPolygons[id];
  }

  final Set&lt;String&gt; _polygonIdsToRemove =
      prevPolygonIds.difference(currentPolygonIds);

  final Set&lt;Polygon&gt; _polygonsToAdd = currentPolygonIds
      .difference(prevPolygonIds)
      .map(idToCurrentPolygon)
      .toSet();

  bool hasChanged(Polygon current) {
    final Polygon previous = previousPolygons[current.id];
    return current != previous;
  }

  final Set&lt;Polygon&gt; _polygonsToChange = currentPolygonIds
      .intersection(prevPolygonIds)
      .map(idToCurrentPolygon)
      .where(hasChanged)
      .toSet();

  polygonsToAdd = _polygonsToAdd;
  polygonIdsToRemove = _polygonIdsToRemove;
  polygonsToChange = _polygonsToChange;
}</code></pre>
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